1. Field of the Invention
The present invention relates generally to receiver protectors, and more particularly to a device for use as a receiver protector in radar systems.
2. Discussion of the Related Art
Receiver protectors in radar systems operate to limit the amount of power reflected toward the receiver from the radar antenna during the time the transmitter is sending out high power signals. When the transmitter ceases to transmit energy pulses, an echo from the target is received by the antenna and passes through the receiver protector. Waveguide receiver protectors operate at various frequency ranges, depending on the size of the waveguides. A conventional receiver protector may consist of a sealed quartz capsule containing a halogen gas, which is located adjacent to an iris plate in the waveguide. A high energy microwave pulse of a predetermined threshold level ionizes the gas in the capsule and causes the high energy pulse to be reflected.
A conventional low threshold receiver protector is comprised of an rf oscillator, a coupling circuit, a high Q microwave cavity and an auxiliary "keep alive" or discharge gap. The "keep alive" or discharge gap provides a constant supply of free electrons to achieve breakdown of gas at low microwave power and a fast recovery time of about 200 nanoseconds to protect the receiver. Fast recovery time performance is desired to detect close range targets by the receiver. The low power oscillator in a conventional receiver protector transmits rf energy into a microwave cavity to create a continuous gas discharge in an auxiliary gap. The electrons from the auxiliary discharge gap drift into the main waveguide discharge gap to provide the seeding necessary to initiate a gas discharge or breakdown in the presence of high power rf pulses. The presence of gas discharge causes a short circuit in the waveguide, therefore, reflecting most of the incident power and acts as a protector.
The tuning, testing and fabrication of such a conventional device is complex. For example, to create the auxiliary discharge, the fixed frequency oscillator needs to be tracked with the cavity frequency when the temperature varies from -40.degree. C. to 70.degree. C. The oscillator, the coupling circuit and the cavity add to the material cost of the main waveguide capillary stage. In addition, such low threshold devices require an external power source to operate the oscillator.
Receiver protectors have been developed which do not require an external power source in order to breakdown the gas. Such receiver protectors are referred to as passive receiver protectors. To achieve a fast recovery time, however, these prior art passive gas stage receiver protectors utilize nickel substrates with radioactive tritium film. The tritium radioactive film on the nickel substrate served to provide a source of free electrons to initiate the breakdown of the gas. To achieve fast recovery time performance, however, H.sub.2 O vapor, as well as heaters to enable the H.sub.2 O vapors to perform at temperatures below 0.degree. C. was used.
Chlorine gas is known to be highly reactive. As such, chlorine has high affinity for electrons; therefore, it attaches to electrons in gas discharge rapidly. This results in a fast recovery time performance.
Conventional passive chlorine gas stage receiver protectors use a radioactive promethium priming of about 5 microcurie strength. A liquid promethium compound is evaporated on the inside of a quartz tube and supplies free electrons for initiating the gas discharge. Due to the weak radioactive strength of the promethium and the low Q of this receiver protector, however, this chlorine gas stage receiver protector has a high firing threshold. A low firing threshold is important in order to achieve low spike and flat leakages at the output of the device. To achieve a low firing threshold of less than 20 Watts, however, a strong radioactive source is needed to utilize the fast recovery time performance provided by a chlorine gas stage receiver protector. A typical titanium titride film of 100 millicurie emits sufficient free electrons to breakdown chlorine gas under 20 Watts of microwave power. The technology of depositing titanium titride film on nickel substrates or rods is well known. However, nickel is reactive to chlorine gas and the use thereof results in gas clean-up and greatly reduces the lifetime of the receiver protector. In light of the foregoing, there is a need for a low cost, less complex, low threshold receiver protector which provides for a fast recovery time and low leakages.